As an example of achieving miniaturization of a semiconductor device, a trench gate structure in which a gate electrode is formed in a trench is employed. As the gap between trench gates is narrowed, a gate density per unit area is increased and an advantage of reducing an ON-state resistance is also increased.
On the other hand, when the gap between the trench gates becomes narrow, there is trouble for controlling the threshold of the semiconductor device and for securing a short-circuit resistance. It is important for the semiconductor device to achieve the miniaturization and an improvement in characteristics while securing the controllability and the resistance using the gate.
For example, having conductivity modulation of a high resistance semiconductor layer, a bipolar power device (a thyristor, a pn diode, an IGBT, an IEGT, a bipolar transistor, and the like) is realized with the low ON-state resistance, and is secured with a large current-carrying capability. However, for the conductivity modulation, the device is hard to make the current flow when a drain voltage (Vd) is equal to or below a built-in potential (Vbi). The bipolar power device is advantageously operated at Vd>Vbi.
On the contrary, a unipolar power device (a MOSFET or the like) has no limitation caused due to the built-in potential, and the current flows linearly in proportion to the drain voltage. However, when the drain voltage is equal to or greater than the built-in potential, the effect of the conductivity modulation cannot be used, and there is no large current-carrying capability. The unipolar power device is advantageously operated at Vd<Vbi.
There is required a device which can realize the characteristics of each of the bipolar power device and the unipolar power device, that is, maintaining a good current-carrying characteristic of the bipolar power device at Vd>Vbi, and achieving an ON-state characteristic lower than a normal MOSFET at Vbi>Vd>0V.